Add trace metrics: event counts, execution duration, and per-type child spans

[pedroamaral-cvfx]

Summary

Enriches OpenTelemetry spans with additional observable data to support monitoring and Datadog timeseries dashboards for Commanded CQRS applications.

Problem

The existing instrumentation creates spans for command dispatch, aggregate execution, event handling, and process management — but the spans lack quantitative data needed for operational dashboards:

• No execution timing as a queryable attribute — span timestamps capture duration implicitly, but an explicit attribute is needed for easy filtering/aggregation in tracing backends
• No event type names in aggregate spans — commanded.event_count existed but without the actual type names
• No per-event-type breakdown — Datadog's "Generate Metrics from Spans" operates on spans (not span events or array attributes), so a list attribute like commanded.events cannot produce per-type timeseries

Changes

1. Execution duration on all spans (commanded.execution.duration_ms)

All four instrumentation modules (aggregate, application, event_handler, process_manager) now capture the telemetry measurements[:duration] and set it as commanded.execution.duration_ms (converted from native time units via :erlang.convert_time_unit/3). A shared native_to_ms/1 helper was added to OpentelemetryCommanded.Util.

2. Event/command type name lists as span attributes

• Aggregate spans now include commanded.events — a list of event type name strings
• Process manager spans now include commanded.commands — a list of dispatched command type name strings

3. Per-type child spans for Datadog metrics

For each domain event emitted by an aggregate, a zero-duration child span commanded.event.emitted is created with commanded.event_type and commanded.aggregate_uuid. For each command dispatched by a process manager, a commanded.command.dispatched child span is created with commanded.command_type and commanded.process_uuid.

Metric	Datadog Query
Total events/time	count(*) where @span.name:commanded.event.emitted
Events by type/time	count(*) grouped by @commanded.event_type
Commands by type	count(*) grouped by @commanded.command_type

New span attributes

Attribute	Span	Type
commanded.execution.duration_ms	All	integer
commanded.events	Aggregate	string[]
commanded.commands	Process Manager	string[]

New child spans

Span name	Parent	Attributes
commanded.event.emitted	commanded.aggregate.execute	commanded.event_type, commanded.aggregate_uuid
commanded.command.dispatched	commanded.process_manager.handle	commanded.command_type, commanded.process_uuid

Test plan

• mix test — all existing and new tests pass
• Aggregate success: child span with correct parent and event type
• Aggregate error: no child spans emitted
• Process manager success: child span with correct command type
• Duration attribute present on all span types

[pedroamaral-cvfx]

this was done on the phone, just a quick chat where I asked how could we make some metrics available through tracing

[pedroamaral-cvfx]

disregard this comment, just fixing the PR list view